Physiological signals have been monitored for a variety of purposes. For example, ECG monitoring involves using sensing electrodes that will detect the electrical currents of the heart so they can be amplified and recorded electronically. Electrodes can be placed inside the body, under the skin, on the surface of the skin, and in the vicinity of the skin. It is often useful to obtain long term (e.g., more than a few days) recordings from ambulatory subjects. This allows recording of transient events such as arrhythmias that may only occur infrequently or under rare circumstances.
Long term ambulatory ECG monitoring is difficult to achieve without the use of an implantable monitoring device. Electrodes have often been very uncomfortable or may induce an allergic reaction, resulting in poor patient compliance. Monitoring systems incorporated into comfortable garments have provided poor quality signals; therefore, the information they provide is not adequate to facilitate good decision making as to the presence or absence of arrhythmias or other abnormalities in the ECG.
Regarding sensing electrodes, one type is a “wet” electrode that is affixed to the skin with an adhesive, and often employs a gel or electrolyte to improve electrical contact with the skin. Wet electrodes have produced high quality signals, relative to other types of electrodes available for monitoring ambulatory subjects. However, these electrodes often induce an allergic reaction when used for more than a few days, may require that the area where the electrode is applied be shaved, and are seldom tolerated by patients for more than two weeks.
Another type of sensing electrode is a “dry” electrode made of a conductive material that is positioned on the skin without using a “wet” contacting material. Although these electrodes are usually better tolerated for longer term recordings than wet electrodes, they often provide a relatively poor quality signal. Large baseline wander and loss of signal due to clipping is common with these electrodes.
In addition to issues associated with electrode design, the data collection instrumentation and signal processing systems used to collect and process signals from dry electrodes have been incapable of addressing the signal quality (e.g., baseline wander) limitations of dry electrodes. Dry electrode systems are typically only used for monitoring heart rate because of these limitations and are not considered useful for collection of diagnostic information such as arrhythmia detections.
These and other considerations have presented challenges to the detection of physiological signals, such as ambulatory ECG signals, for a variety of uses such as the assessment of arrhythmias and intervals.